Bearings
dave williams
rec.autos.tech 08-27-93
- Interestingly, back in the 1920s and 1930s, roller lifters and
rockers were quite common, particularly on better-brand cars. Tubular
manifolds weren't unknown either.
- The rollers were used because the lubricants of the day weren't good
enough to provide the longevity the designers wanted. Once high
pressure additives became available, designers specified better oil and
went to the cheaper flat tappets.
[email protected] (Dave Williams)
gnttype 28 Jan 1994
- -> There was an article in the GSXtra a couple of issues ago discussing
-> this for the 455 motors. The basic conclusion was that it was better
-> to cross drill than to loose the bearing surface.
- Correct. If one extra hole is going to make a difference in crank
strength, the crank was too weak to begin with.
- Grooved bearings are a problem in that a sleeve bearing is more
dependent on width than diameter. When you groove the bearing, you have
two very narrow bearings instead of one wider one, and the load capacity
goes down drastically.
[email protected] (Dave Williams)
gnttype 28 Jan 1994
- -> Do you really lose load capacity with grooved bearings?
- Yes, and the amount is substantial. Do you have a Machinery's
Handbook? If not, the library might. The section on plain bearings is
quite interesting.
- Basically, the load isn't distributed evenly across the bearing. The
center carries most of the load, with the load dropping off at the
edges, where oil is bled away.
- -> The old GS455s apparently used fully grooved bearings.
- Lots of engines did and do. That's because the connecting rods are
loaded more than the crank, and they're running on narrower, smaller
diameter bearings too! The designers were trading some main bearing
load for better lubrication for the rods.
- -> Would it improve crank strength to drill it at 120 degree spacings,
-> i.e. three holes, rather than 180? That way you wouldn't have a
-> straight "parting line" across the crank. I wonder if anybody has
-> heard/tried this out.
- I've never come across the idea, but it's interesting. With a grooved
upper shell, you have 180 degrees of oil feed. Technically a straight
drilled crank would feed oil to the rods at all times. In actuality,
the oil has to start and stop, and it has a fairly long travel path from
the oil feed point at the shell, through the groove, and around to the
side of the crank where the oil hole is. The hole is moving, which
ain't the best way to ensure steady flow, either.
- With the crossdrilled crank, you never have a hole more than 45 degrees
from the feed, as opposed to 180 max for the single drilled crank. I'd
tend to guess that, if you're having oiling related problems with a
cross drilled crank, a three-drilled crank wouldn't help you any.
[email protected] (Dave Williams)
gnttype 03 Mar 1994
- -> surface and corrosion characteristics. I think the standard for
-> years were the all aluminum alloy type or the babbit coated ones. Go
-> the few extra bucks and get the most up to date in journal bearing
-> technology.
- The aluminum bearings aren't seen much. Some heavy duty truck motors
use them, and Diesels, but the only aluminum you see in a normal car
bearing is aluminum-bronze.
- Chrysler kept with Babbitt bearings in some locations all the way up to
the '60s, but the rest of the industry had changed over to trimetal in
the '50s. Plain bearing design hasn't changed significantly since. You
can futz around with the hardnesses of the layers to trade embeddability
for strength, but that's about it.
- -> Incidentally, for journal bearing prep, I think the moly EP grease is
-> best
- I used to do that, but Chevrolet's Chevy Power manual says not to, as
it will stop up the oil filter. I switched to STP for the bearings
after that, and the bottoms of the lifters, the rocker wear areas, etc.
Everything else gets a squirt from a pump oiler filled with Mobil 1 synthetic.
- -> Most of it dripped off the bearings. The moly is what is in GM's
-> E.O.S. and this is supposedly good prep.
- GM EOS (and Ford's equivalent) have lots of zinc in them as a high
pressure additive. If you're reringing the motor GM says not to use it
on cars equipped with oxygen sensors. I suspect some cam lubes are the
same basic stuff. A few hundred miles probably won't kill the sensor,
though.
- -> this is supposedly good prep. Heard you should change the oil/filter
-> after lik 1 hour though because the moly clogs filters fast.
- That's about how I do mine. The first hour of operation is the worst
the engine is ever going to see. That's when any debris is going to get
circulated around, the cylinder walls are being lapped by the rings, and
all sorts of monkey-motion is going on. $10 for five quarts of oil and
a filter is pretty damned cheap insurance if you ask me.
- -> Is there a way to prime the oiling system in the 3.8 before you crank
-> it over the first time?
- Someone probably sells a pre-oiler, but cranking it over with the plugs
out until oil pressure comes up works okay. Not that I'd turn up my
nose at a pre-oiler if someone offered me one, though.
chaos.lrk.ar.us!dave.williams (Dave Williams)
hotrod 10 Jul 1994
- A couple of years ago discussion touched on 360 degree oiling - instead
of running grooved mains on both top and bottom, use a plain bottom
bearing, cut an oiling groove in the block and cap, and drill holes as
desired in the bottom bearing. John put his hairy moderator Foot on
the thread, something about "don't bring that one up again." C'mon, John.
- [Moi? I plead innocent due to loss of memory.... JGD]
- I built one engine this way, no trouble, just experimenting.
Chevrolet's race book talks about it. That's about all I've seen, so I
missed out on whatever flamewars took place.
- Fully-grooved mains (or grooved cranks, now considered a Very Bad Idea)
were designed to get more oil up to the rod bearings. Most cranks are
drilled diagonally from the main journal up to the rod journal. When
the main journal hole is in the grooved area, oil (hopefully) flows
through the drilling to the rod bearing. Using a fully grooved bearing
lets oil flow all the time, at the expense of losing well over half of
the mains' load carrying capability. Most engines use plain bearings on
the bottom, grooved on the top, and the rods appear to live OK. When
there's question, the crank is usually cross-drilled. A second hole is
drilled, from the original rod oiling hole in the main journal, straight
across to the other side of the crank. When the crank turns so as to
block oil flow to one side of the drilling, the other side comes up and
starts taking oil.
- Now, that plain bottom bearing is only lubricated by what oil has
managed to stay attached to the crank. A few small drilled oil holes
can feed more oil from the bottom, assuming you drill them on the low
pressure side. As the crank rotates it builds up a "wedge" of high
pressure oil adjacent to the point of maximum load. You don't want to
put any oil holes there, or you might wind up with the oil wedge
actually pumping oil right back into the oil galleries.
- Now, if the crank were just a smoothly spinning shaft, oiling from the
bottom wouldn't be much help. Modern theory is that, under load, the
crank is writhing about like a snake, or as much as it can subject to
the constraints of the bearing clearances. So the nice oil flow and
wedge model breaks down when the crank starts to whip and the wedge
moves to the top, or the side, or wherever it wants. So 360 degree
oiling would (theoretically) ensure the crank was always getting some
oil, no matter what was going on.
- Hank the Crank (are they still around?) and Bruce Crower favored
putting *all* the oil in through the main cap, though I've yet to figure
a real good reason for it.
[email protected] (Dave Williams)
gnttype 09 Dec 1995
- -> He showed me the bearings he had out and there was black "SCARRING"
-> towards the center of all of them,
- Hmm. That's not anything I've seen before. Most modern bearings aren't
completely circular for various reasons, so it's common to see 1/4 to 3/8
inch of dark band at the edges where the top and bottom half touch each
other. Worn areas at the center of the bearing would be coppery or silvery,
depending on how many layers of the bearing had been worn.
- If the engine had been run low on oil or detonated a lot, you might see a
darkish area at around 10 or 2 o'clock, on the top shell only. This is
burned oil, and though you'd ordinarily replace the bearing since you're
that far into the motor, it hasn't failed yet.
[email protected] (Dave Williams)
fordnatics 09 Aug 1996
- -> can you explain that a bit? does the heat buildup cause the bearings
-> to expand and press too hard?
- Uh-oh, that's starting to get complex. Stripped to the bare essentials, the
a thin layer of oil "sticks" to the crank and the bearing shell. In between
is more oil which is sliding on the oil stuck to the first two layers.
- The problem is, oil has viscosity, otherwise defined as "internal friction".
The molecules in motor oil are long and stringy, which is good for
lubrication, but the long molecules make a lot of heat sliding around each
other. They also break through plain old mechanical shear -oil *does* "wear
out" despite the claims of miracle filter vendors.
- The higher the loading on the bearing, the more heat the oil makes.
Eventually the bulk oil in the pan gets hot too. Finally it will get hot
enough so the temperature rise in the bearing is enough to cause the oil to
break down very rapidly, char, or even reach flash point. However, before
this happens the oil is actually hot enough to start melting the soft
embeddable surface of the bearing, causing smearing, pits, or a spalling.
- The characteristic of the 302 seems to be, the Ford bearings have lowish
melting point and get wiped out fairly easily. Clearances open up, which
causes more oil to be pumped through, which reduces the temperatures, so
things appear to reach equilibrium somewhere about the time you get to the
copper plating in the bearing shell.
- Practically any amount of oil will do for lubrication. The reason you pump
mass quantities through at 40 to 100psi is for cooling the bearings. And
since the oil runs pretty hot already, you have to pump even more of it to
get the same cooling effect. You could probably chop pump output 'way back
if you could ensure a supply of room-temperature oil; in practice you're
hard pressed to keep it down below 220F in a competition engine.
- If you have big, wide bearings, low RPM, and a low power output, you can get
away with splash oiling. Pre'52 Chevy 216 six cylinders and Briggs&Stratton
lawnmower engines are examples. They have a hole in the bottom of the rod
and a scoop to direct oil to the rod journal. For the speeds and loads they
operate at, it works. Many cars made before WWII had pressure lubrication
to the mains and splash to the rods.
- Bear in mind this is the Readers' Digest Condensed Version. Lubricant design
used to be a branch of chemistry; now it's a subset of a distinct
engineering specialty known as tribology.
[email protected] (Dave Williams)
fordnatics 27 Jan 1998
- -> I thought the reason for feeding the bearings from the bottom is
-> because that i the load rests. I know they are different but all our
-> turbine and generator be fed from the bottom.
- When a shaft turns inside a plain bearing you get a funky seashell-shaped
pressure curve if you look at a polar plot. That is, the pressure across
the bearing is uneven, with the highest point of pressure being about thirty
degrees from the load vector. Leading that high point is the low pressure
point, where it is most advantageous to feed the bearing.
- In an ideal situation you would indeed feed the bearing from the bottom,
just as it is in the Ford camshaft journals. The sum of the load vectors
from the lifters is straight down. Unfortunately most automobile bearings
are out there in the "special case" category. They are *very* narrow for
plain bearings. Plain bearings are sensitive to edge effects - pressure is
lost to the sides. The aspect ratio of all the bearings in a car engine is
so low you are dealing *primarily* with edge effects. So what happens is,
when they inject the oil to the bottom of the bearing, the bearing carries
it along fifteen degrees or so of rotation and then promptly spits it out
the sides rather than building the "hydrostatic wedge" which is what makes a
plain bearing work.
- Thus, it works fine for your turbines, which presumably have properly
designed bearings. But the narrow bearings in a car present a special case.
[email protected] (Dave Williams)
diy_efi 06 May 1998
- -> regenerative braking and massive power switching module. On top of
-> all that, pure water was used for cooling of motor and get this: was
-> fluid used in turbine's journal bearings instead of oil.
- Sure, water will work just fine. Take a look at a big hydroelectric turbine
sometime; chances are it'll have water plumbed to sleeve bearings. Once the
shaft is spinning any fluid will do - even air, which is often used in very
high speed applications.
- Automobiles use oil-fed plain bearings primarily because they already *have*
oil in the crankcase to lube the myriad things which require lubrication, so
it makes sense to use oil.
[email protected] (Dave Williams)
fordnatics 27 Dec 1998
- -> Anyone know why no one uses a desmodromic arraingement like Ducati?
-> No springs at all. Cam opens and closes the valve.
-> No way the valve can float then.
- I've considered this one myself, with no clear answer. Even Ducati was
turning away from desmo last I heard.
- Production costs would probably come out close to the same in OEM-volume
quantities. I suspect it's mostly inertia - coil sprung valves are simple,
well understood, and do an adequate job with few failures.
- I have been predicting a resurgence of desmodromic valvetrains for the last
few years, though. Springs suck some power and cause friction; as
automakers writhe under the screws of emissions and fuel mileage
legislation, desmo valves will surely return, like other "obsolete"
technologies - four valve heads (over a century old now), coil-on-plug
(Fordson tractors used them before WWII), multiple spark ignition (Model T
Ford) etc.
- We'll probably also see more use of ball and roller bearings. Plain
bearings are more efficient than roller bearings, *but* they require
pressure fed oil. Unfortunately a plain bearing's lubricant flow
requirements are opposite the way oil pumps work - they need more flow at
lower RPM, less at high. Oil pumps are sized to give full flow at idle;
past that they dump excess oil overboard, heating and aerating it. We're
talking 5-15hp here, plus the benefits for engines like the 4.6 Ford, which
have a horrible oiling system to start with. Cam journal wear is the #1
warranty problem on the 4.6. Torrington Corp. has been trying to sell Ford
on roller cam bearings for years - the rollers would live fine in the
crankcase oil mist, no pressure feed required - but Ford evidently figures
it's cheaper to quietly swap whole motors rather than fix the problem or its
symptoms. The symptom is journal wear; it can be cured by rollers. The
problem is the long and torturous oil path from the sump, through the pump,
to the journals; it's a design flaw that would be harder to correct than
going to rollers.
[email protected] (Dave Williams)
fangle 09 Nov 2000
- Most of the information I normally use on plain bearings came from
Machinery's Handbook, which has a sizeable section in the design of plain
bearings. Rogowski's "Elements of the Internal Combustion Engine" is in
line with that, but it has some engine-related data:
- "The following data were taken on an eight-cylinder engine with a
displacement of 221 in.^2, by motoring and firing at 2,000 RPM:
- hp required to motor engine with pistons removed = 1.5
- hp required to motor engine with pistons in place but cylinder
head removed = 11.5
- hp required to motor with engine completely assembled = 16.0"
- That's likely a flathead Ford V8. The losses for "pistons removed",
assuming the rest of the engine was left alone, is much less than other
sources have indicated, while piston friction is much higher. The usual
figure bandied about is 50% of the friction is in the pistons, the rest
scattered about the rest of the engine.
- FYI, YMMV, eieio.
[email protected] (Dave Williams)
fangle 10 Nov 2000
- -> Was this with or without cam and lifters?
- I typed it in exactly as it was printed in the book. "What you see is what
you get," as Flip Wilson used to say. I wondered about the cam issue
myself... but the flatheads only used 60 or 70 pounds of spring pressure
when open; no need for lots when you just have a 1.5" valve, a retainer, a
hollow lifter, and 1/3 of spring weight.
[email protected] (Dave Williams)
fangle 10 Nov 2000
- -> Wayne now mentions lubricant flow rate, bearing clearance, bearing
-> load capacity - ah!!!, this was not found in my eleven months of
-> intensive crankshaft research, using Dave Williams' and Robert
-> Harris' 'usual sources'
- Did you ask? Machinery's Handbook, Marks' and Kent's Handbooks, and of
course Rogowski, and (need I belabor the point) half a dozen papers at
naca.larc.nasa.gov.
- All automotive sleeve bearings have such short aspect ratios they're running
mostly in the "edge effect" zone instead of a normal sleeve bearing. Other
than that, any journal large enough not to allow excessive crank flex seems
to be fine.
[email protected] (Dave Williams)
fangle 11 Nov 2000
- -> information nowhere else - and I have just near zero interest in 1920
-> drillpress plain bearings (is this wrong?).
- Wrong. Plain bearings are plain bearings. The information in Marks' and
Kent's is highly condensed but covers most applications, including internal
combustion engines. You have to remember they're generally working with
different 'service factors' (same as electric motor ratings) than car guys
are used to; their recommended formulas will produce bearings the size and
shape of soup cans for a small block V8.
- -> Elaborate on "edge effect", please? (I'll be looking at those books,
-> but it will take time to find them!).
- The rotating member floats off-center in the bearing on an oil wedge built
by its own rotation. At the sides, it's easier for the oil to squirt out
the side than to maintain the wedge, so the edges carry very little load.
If a bearing is too narrow the bearing is depending mostly on the oil's
internal lubricity and its load rating goes way down. That's why a fully
grooved main bearing has less than half the load capacity of an ungrooved
bearing.